The present invention relates to a process and an apparatus for separating air by cryogenic distillation.
In order to produce liquid from a double air-separation column, it is known to use two turbines in series in order to expand the air.
U.S. Pat. No. 3,905,201 describes an air separation process which produces liquid nitrogen and liquid oxygen, using two turbines in series; the second turbine expands air, which is then either vented to atmosphere or is sent to the low-pressure column.
EP-A-0,420,725 describes a system- in which a Claude turbine produces air intended for the medium-pressure column and air which is warmed before being expanded and either vented to atmosphere or sent to the low-pressure column.
EP-A-0,542,539 describes an air separation process using two air turbines in series, the second of which is a blowing turbine.
The outlet temperature of the first turbine is also at the inlet temperature of the second turbine.
EP-0,641,983 describes an air separation process using two air turbines in series with the air from the first turbine being warmed before the air is sent into the second turbine.
The object of this invention is to reduce the specific energy or the air separation process, by eliminating as far as possible the deviations from the exchange diagram of the main heat exchanger.
According to the invention, a process for separating air by cryogenic distillation is provided in which a first air stream is cooled, at least part of the first cooled stream is expanded in a first turbine, the stream expanded in the first turbine is mixed with a second air stream in order to form a third stream, at least part of the third stream is expanded in a second turbine, at least part of the stream expanded in the second turbine is introduced into a rectification column of a double column, the air in the double column is separated into oxygen-rich and nitrogen-rich fluids and a fluid in liquid form is produced as the final product.
According to other aspects of the invention,
the stream expanded in the first turbine is cooled before it is expanded in the second turbine,
the stream expanded in the first turbine is smaller than the stream expanded in the second turbine,
at least part of the stream expanded in the second turbine is introduced into the medium-pressure column or the low-pressure column,
the stream intended for the first turbine is at a pressure of at least 15 bar,
the second air stream is at a pressure of at least 6 bar,
the stream expanded in the second turbine consists of at least 70% air,
the pressure at the outlet of the second turbine is slightly above the pressure in the medium-pressure column,
the inlet temperatures of the first and second turbines are intermediate temperatures of the main exchanger,
part of the first stream and/or part of the third air stream are/is cooled further in the main exchanger and are/is liquefied at the cold end,
at least one optionally pressurized liquid stream vaporizes in the main exchanger,
a stream of fluid from the low-pressure column of the double column feeds as argon column and an argon stream is drawn off from the latter,
the stream expanded by the second turbine leaves it as a two-phase fluid.
According to another aspect of the invention, an apparatus for separating air by cryogenic distillation is provided, which comprises;
a main heat exchanger;
a double air-distillation column;
means for sending an air stream into a first turbine;
means for mixing the air expanded in the first turbine with a second air stream in order to form a third air stream;
means for sending at least part of the third air stream to a second turbine;
means for sending air expanded in the second turbine to the double column; and
means for drawing off a liquid product from the double column.
According to other aspects of the invention, an apparatus is provided which comprises means for cooling the third air stream before at least part of it is sent to the second turbine.